You won't gain more than 2-3mb by removing the gui, but if you still wish to do so, the biggest parts are Xvesa, jwm, X libs, and libfltk. Then there are the fltk guis, but they are only a couple of kb each.
How to add an extension to the iso:
just create a new directory "tce" right beside the directory "boot", so it's at the top level of the to-be-iso, and put extensions into that tce directory.